The invention concerns a process and apparatus for the separation and dewatering of solids contained in waste waters in low concentrations of less than about 10%. Such solids may originate in animal intensive operations or in industrial processes for processing plants or animals or their parts, and mixed with fibers, for example, animal hairs or vegetable fibers.
In intensive animal husbandry the use of liquid manure removal has been found useful and is being used increasingly. A report of the Technological Institute of the Federal Research Institution for Agriculture, Braunschweig-Volkenrode, published in Grundl. Landtechnik, Vol. 37 (1987), No. 3, pg. 98-107, by K. Krause and R. Ahlers, describes the problem of liquid manure removal and the importance of the solid-liquid separation of the liquid manure, under the title of "Process Technology of the Separation of Liquid Manure". The advantages and disadvantages of individual devices are described and compared with each other.
One of the simplest machines for the separation of solids and liquids is the screw press or screw press separator wherein the screw turns and the housing interior become narrower toward the discharge for achieving a gradual compression of the solids being separated. The build up of the pressure by means of such narrowing is described in DE 26 36 372 and DE-OS 29 11 899. However, the operation of these arrangements in actual practice showed that all of the narrowing sections have a detrimental effect on the throughput, the field of application and the flexibility of the screw press separator. Whenever the quantity of the introduced suspension or its consistency varies, for example due to a variation of the content of fibrous solids, the solids are compressed in the narrowing (conical) areas, with the result that the press separator becomes clogged.
In DE-OS 27 00 542 and in DE 29 02 432 conical parts are eliminated from the screw press separator described. Only the passage through the housing wall to a strainer baskets of reduced diameter, so that this piece of the wall can build up a certain counter pressure. Although in this separator, with the narrowing that is to be considered ineffective, the uniform screw driven toward the discharge does dewater the suspension, there occurs no compression effect to obtain a high proportion of the dry substance, for example over 20% in pig liquid manure.
The object of the present invention is to broaden the field of application and improve the flexibility of the afore-described types of screw press separators, and increase the dry substance content of the solids.